1. Field of the Invention
The present invention relates to a photo-alignment-method-applied liquid crystal display device and its manufacturing method.
2. Description of the Related Art
Commonly, display by a liquid crystal display device is implemented by applying voltage to liquid crystal molecules in a liquid crystal layer sandwiched between a pair of substrates so as to change the orientation of liquid crystal molecules and by utilizing the resultant change in the optical properties of the liquid crystal layer. Liquid crystal display devices provided with a switching device such as a thin film transistor on a pixel-by-pixel basis, which are commonly referred to as active-drive-type liquid crystal display devices, are conventionally represented by the Twisted Nematic (TN) display type. This type has electrodes formed on each of the two substrates forming a pair sandwiching the liquid crystal layer such that voltage is applied to the liquid crystal layer substantially perpendicularly to the boundary faces between the substrates and the liquid crystal layer and implements display by utilizing the optical rotatory effect of liquid crystal molecules constituting the liquid crystal layer. The largest problem with liquid crystal display devices of this TN type is their narrow viewing angles.
Also known is the IPS type which has inter-digital electrodes formed on one of a pair of substrates so that an electric field substantially in parallel with the surfaces of the substrates is generated to implement display by utilizing the birefringence of the liquid crystal layer, that is, by rotating liquid crystal molecules constituting the liquid crystal layer in a plane substantially parallel with the substrates. Due to the in-plane switching of liquid crystal molecules, this IPS type has a wider viewing angle, lower load capacitance and other advantages over the TN type. Considered promising as a new liquid crystal display device which may replace the TN type, the IPS type is recently making rapid progress. Disclosed in Patent JP-A-9-73101 is an IPS type which attained improved transmittance by using a transparent conductive film to form one or both of a pair of electrodes to apply an electric field to the liquid crystal layer.
Due to superior viewing angle characteristics (luminance contrast ratio and tone/color inversion) and bright display, the IPS type liquid crystal display device (hereinafter, denoted as “IPS-TFT-LCD”) is a promising technology for monitors and televisions having wider display areas. In the IPS-TFT-LCD, orientation control films given the capability to control the orientation of liquid crystals are formed on the respective interfaces of the liquid crystal layer with the pair of substrates which sandwich the liquid crystal layer. In this connection, however, it is still difficult to practically manufacture 20-inch or larger size IPS-TFT-LCDs (large panels) unless a new structure or process is developed.
In the case of the IPS-TFT-LCD, it is especially difficult to impart uniform alignment treatment to the whole area of the large orientation control films because of many steps in contact with the liquid crystal layer. As compared with the conventional TN type and in particular with the currently-popular normally-open TN type (bright with low voltage and dark with high voltage), the margin allowed for the alignment treatment of the orientation control films is remarkably narrow. This narrow margin is attributable to the following three points (1) to (3).
(1) Stepped Structure
Due to the principle, the IPS-TFT-LCD is required to have a number of about-several-μm-wide, long, thin electrodes (sometimes referred to as inter digital electrodes) disposed therein. Therefore, fine step structures are formed. Although dependent on the thicknesses of the electrodes and the geometries of various films formed thereon, their heights usually exceed 10 nm. In a high-transmissivity pixel structure, a thick inorganic insulating film is formed, and its surface has a certain level of planarity regardless of the surface irregularities of the layers below it. Thus, in the high-transmissivity pixel structure, the steps (surface irregularities) of the orientation control film are mainly attributable to the top electrode layer. Over these steps, an orientation control film (also referred to as an alignment film) made of polyimide or other polymer is formed.
In the conventional mass-production technology, a rubbing process is performed on this orientation control film in order to impart a liquid crystal alignment (initial alignment) ability to the film. The rubbing cloth comprises 10-to-30-μm diameter, thin fibers bound together. Substantially, the liquid crystal alignment ability is imparted to the alignment film as a result of each fine fiber giving a certain directional shearing force locally to the film. Although very thin fibers in the order of several microns are available, such very thin fibers are not practically used for the rubbing alignment since rigidity is needed to give a certain level of frictional force. In the IPS scheme, since the inter-electrode space ranges approximately from 4 to 30 μm and therefore is substantially the same as or smaller than the fiber diameter, poor alignment is likely to occur around the steps due to insufficient rubbing. This poor alignment results in lower image quality since it lowers the black level (blackness) and consequently lowers the contrast ratio and lowers the luminance uniformity.
(2) Alignment Angle
Due to the principle, the IPS-TFT-LCD is required to set the initial alignment direction deviated from the longitudinal direction of electrodes or from the direction perpendicular to that longitudinal direction by a certain angle. Here, the electrodes refer to signal line electrodes, common electrodes within pixels, and pixel electrodes. To define the initial alignment direction by rubbing, it is necessary to rub the alignment film with 10-to-30-μm fibers in a direction inclined at a predetermined angle as described above. However, fibers tend to be dragged along the edges of steps formed due to wiring lines extending in a particular direction, such as signal line electrodes, common electrodes within pixels, and pixel electrodes. This disturbs the alignment, resulting in a shallower black level and other disadvantages in image quality.
(3) Deepening of Black Level
One of the characteristics of the IPS-TFT-LCD is its superior in deepening the dark level (black display). Accordingly, disorder in the alignment is likely to be visually noticeable as compared with other types. In the conventional normally-open TN type, a dark level is attained when high voltage is applied. In this case of the high voltage, almost all liquid crystal molecules are oriented in the direction of the electric field perpendicular to the substrates. The dark level is obtained by the relationship between the alignment of the liquid crystal molecules at high voltage and the arrangement of polarizers. Thus, theoretically, the dark level uniformity is not much subject to the initial state of orientation at low voltage. Further, human eye are sensitive to changes of the black level since they perceive luminance unevenness as a relative ratio of luminance and its perception reacts substantially on a logarithmic scale. In this respect, the conventional normally-open TN type has an advantage since liquid crystal molecules are forcibly oriented to one direction with high voltage irrespective of the initial state of orientation.
In the case of the IPS type, since the dark level display is produced at low or zero voltage, the IPS type is sensitive to the disorder of the initial orientation. In particular, if liquid crystal molecules are homogeneously oriented such that they are parallel to the upper and lower substrates and if polarizers are arranged such that the optical transmission axis of one polarizer is parallel to the orientation direction of the liquid crystal molecules and that of the other polarizer is orthogonal to that orientation direction of the liquid crystal molecules (called birefringence mode), polarized light incident on the liquid crystal layer travels without being linearly disturbed almost at all. This is effective in deepening the dark level.
In the birefringence mode, transmittance T is commonly given by the following equation.T=T0·sin2{2θ(E)}·sin2{(π·deff·Δn)/λ}
In the equation, T0 is a coefficient determined mainly by the transmittances of the polarizers used in the liquid crystal panel; θ(E) is the angle between the orientation direction of liquid crystals (effective optical axis of the liquid crystal layer) and the transmission axis of polarized light; E is the intensity of an applied electric field; deff is the effective thickness of the liquid crystal layer; Δn is the anisotropic refractive index of the liquid crystals; and λ is the wavelength of the light. The product of the effective thickness of the liquid crystal layer, deff, and the liquid-crystal anisotropic refractive index Δn, i.e., deff·Δn, is called retardation. Note that the liquid crystal layer's effective thickness deff is not the whole thickness of the liquid crystal layer but the partial thickness of the liquid crystal layer where the orientation directions of the liquid crystal molecules actually change when voltage is applied. This is because liquid crystal molecules near the boundary faces of the liquid crystal layer do not change their orientation direction due to anchoring effects near the boundary faces even when voltage is applied. Thus, the whole thickness dLC of the liquid crystal layer sandwiched by the substrates is always larger than deff, i.e., deff<dLC. This difference can be roughly estimated to be from 20 nm to 40 nm although dependent on what substances respectively constitute the liquid crystal layer and the alignment films in contact with the liquid crystal layer.
As is obvious from the above equation, only the term sin2{2θ(E)} of the equation is dependent on the electric field intensity. The luminance can be adjusted by changing the angle θ according to the electric field intensity E. Operation of the normally-closed type is sensitive to the disorder of the initial alignment since the polarizers are arranged such that θ is 0 degrees when voltage is not applied.
Thus, uniform alignment is very important for the IPS type. Accordingly, problems of the conventional rubbing method have come to the fore. Generally, the rubbing alignment has many problems with its rubbing process, including not only damage to TFTs by frictionally charged electricity and defective display due to poor alignment attributable to the disordered fiber ends of the rubbing cloth and dust but also the necessity to frequently replace the rubbing cloth. In order to solve these problems of the rubbing alignment process, various methods capable of imparting the liquid crystal aligning properties without rubbing, which are commonly referred to as “rubbing-less” alignment methods, have been studied and proposed. Among them is the photo-alignment technique which irradiates a polymer film with polarized ultraviolet light or the like to give the film the ability to align liquid crystal molecules without conducting the rubbing process.
An example of this technique is disclosed in Gibbons et al., “Nature,” Vol. 351, p. 49 (1991). Not needing the conventional rubbing process, this technique can impart the liquid crystal aligning ability to a film by irradiating it with polarized light. Unlike the rubbing method, this photo-alignment method is free from film surface damage, static electricity and other problems. In addition, its simplicity as a manufacturing process is advantageous in terms of industrial manufacturing. Accordingly, this method is gathering attraction as a new method for giving the liquid crystal aligning ability without performing the rubbing process.
As the material for the liquid crystal alignment film, it is proposed to use a polymer compound having a photoreactive group introduced to side chains thereof since the material must be photochemically sensitive to polarized light. Polyvinyl cinnamate may be cited as a major example. In this case, dimerization at the side chains caused by irradiation is thought to produce anisotropy in the polymer film, thereby aligning liquid crystals. It is also proposed to distribute a low-molecular-weight dichroic azo dye in the polymer material and irradiate the film surface with polarized light to create the liquid crystal aligning ability in the film. Further, it is reported that a specific polyimide, if irradiated with polarized ultraviolet light or the like, aligns liquid crystal molecules. The liquid crystal aligning ability in this case is considered attributable to depolymerization of polyimide main chains in a fixed direction by irradiated light.